RE: XAUI AC coupling
Boy - you look away for a minute and this thing goes off the rails.
First, let me say that the proposed DC bias point of 0.75V is in my view
decidedly nonoptimum and wastes power. Even if an optimum point for some
particular technology *were* chosen, it would be nonoptimum in other
technologies or technology generations.
While I don't find this in the email thread, I seem to remember from New
Orleans that the impetus for supporting DC coupling comes from backplane
applications. I think it's a mistake to create a standard where
implementations of the standard are permanently penalized in order to
support some reuse of the silicon in non-standards based applications. If
I've misinterpreted the reason for this initiative, my appologies.
Now to the point: Any IC manufacturer, faced with a "recommended" common
mode specification, will be obliged to meet it in order to not be frozen out
of applications where DC coupling was chosen as a system implementation.
All silicon guys try hard to superset known specifications whenever
possible. So, while making DC coupling "recommended" or "optional" may make
it more palatable to some, it is really just a ruse. It will have the same
impact as a mandatory specification in terms of the behavior of suppliers.
So, we will be mandating a solution that wastes power in at least some of
the technologies people will use to build these things, even for cases where
AC coupling *is* used, since we will have to assume it is not. There's no
way we'll build two output buffers, or have some kind of mode switch, or
have two different versions of the chip. Everyone will bear the power
and/or margin penalty, whether they AC couple or not.
I vote that we require AC coupling, and that no DC voltages are specified or
Joel H. Dedrick
Optical Networking Division